A typical metallurgical furnace is a container having sidewalls with a multi-layer construction. The outer layer is typically a steel shell provided for structural support. The inner layer includes a refractory lining, constructed from one or more layers of refractory bricks, that is provided to shield the outer steel shell from molten materials and aggressive chemicals inside the furnace. In some furnaces, a cooling layer is also provided between the outer steel shell and the refractory lining to prevent excessive heat transfer from the refractory lining to the outer steel shell. In some furnace designs, the layers of brick and/or cooling elements are set in place with a soft sand-like material that solidifies during the operation of the furnace.
During the operation of a metallurgical furnace, the refractory lining is deteriorated by mechanical and thermal stress in addition to chemical corrosion resulting in a loss of overall refractory lining thickness. As the refractory lining deteriorates molten materials and aggressive chemicals penetrate into widening spaces in and/or between refractory bricks leading to delamination (i.e. separation) of the layers in the refractory lining. Deterioration of the refractory lining ultimately leads to structural failures that may cause the outer steel shell to be exposed to molten materials and aggressive chemicals inside the furnace. Moreover, if the molten materials and aggressive chemicals reach the outer steel shell there is an imminent risk of severe injury to personnel working near the furnace, because the outer steel shell is not capable of reliably holding back the molten materials and aggressive chemicals from inside the furnace. Loss of heat transferability and conductivity are also known to occur as results of the deterioration of the refractory lining.
Another mode of refractory lining deterioration, common in furnaces that include water-cooled elements, is hydration of the refractory lining. Under certain temperatures, water that has leaked from a cooling element can react with the refractory bricks causing expedited deterioration of the refractory lining. In particular, magnesium (MgO) based refractory bricks are susceptible to this mode of failure.
It is desirable to regularly check the thickness of the refractory lining, as well as inspect the refractory lining for defects such as cracking, delaminations, accretions and other build-up. Making a reliable and accurate assessment of the refractory lining thickness is difficult to do without first emptying the furnace and shutting down the industrial process in which the furnace is involved. Shutting down a metallurgical furnace for routine inspection is costly and operators try to make use of inspection methods that can be employed while the furnace is operating. However, the hostile working-environment, that the furnaces are included in, skews the measurements made. For example, extremely high temperatures in the furnaces, vibrations, ambient noise, dust, and electrical and mechanical hazards are known to distort the thickness measurements generated by the previously known inspection methods. A systematic method of taking such sources of error into account has not been developed to improve previous inspection methods. As a result, operators are forced to shut down and cool furnaces in order to check the refractory lining from time-to-time.